Muscle Protein Synthesis (MPS)
The cellular process that builds and repairs muscle
Plain English
Muscle Protein Synthesis (MPS) is the process by which your body builds new muscle protein from amino acids. It is triggered by two distinct stimuli that work synergistically: resistance training and protein intake, specifically the amino acid leucine. MPS must exceed muscle protein breakdown (MPB) for net muscle growth to occur, which is why both training and nutrition are required and neither is sufficient alone.
The Mechanism
MPS is controlled primarily by the mTOR (mechanistic Target of Rapamycin) pathway, a signaling complex inside muscle cells that acts as the master regulator of protein production. Two independent inputs activate mTOR: mechanical tension from resistance training (via growth factors and the IGF-1 pathway) and leucine from dietary protein (via the Rag GTPase complex on the mTOR surface).
Leucine activates mTOR at a threshold concentration, roughly 2.5 to 3 grams per meal. Below this threshold, protein intake does not fully trigger synthesis. Above it, you reach a ceiling where additional leucine provides no marginal benefit. This is why meal protein doses of 30 to 40 grams matter more than total daily protein spread across small amounts: a 10-gram protein snack does not reach the leucine threshold regardless of daily totals.
MPS is elevated for approximately 24 to 48 hours after a resistance training session in most individuals, though the window is shorter in trained athletes (closer to 24 hours). This duration is part of why training frequency matters: muscles trained once a week are in a synthesis-elevated state for at most 2 of 7 days. Training each muscle group twice weekly doubles the total synthesis-elevated time, which is why 2 sessions per muscle per week is the evidence-based minimum for hypertrophy (Schoenfeld et al., 2016).
Why It Matters
Muscle grows between sessions, not during them, and only when protein and training both send the signal.
MPS is the cellular mechanism behind all muscle growth and repair. Understanding it shifts your nutrition strategy from "eat more protein" to "hit the leucine threshold at each meal and time protein around training." Pre-sleep protein (30 to 40g casein) captures an otherwise wasted overnight synthesis window (Res et al., Maastricht University, 2012). Protein before bed does not disrupt sleep; it extends the muscle building signal through the night when training-induced growth hormone is highest.
Common Misconception
The common belief is that protein immediately after training is critical and that the window closes within 30 minutes. The research picture is more nuanced. Aragon and Schoenfeld (2013) showed the anabolic window extends 4 to 5 hours, and total daily protein at adequate leucine threshold doses matters far more than post-workout timing. The case for post-workout protein is real but modest; the case for consistent 30-40g meals throughout the day is stronger.
Signs It Is Disrupted
- Muscle mass decreases despite consistent training and adequate calorie intake
- Recovery feels incomplete between sessions even with normal sleep
- Strength gains stall for more than 3 to 4 weeks without a change in training stimulus
- High protein intake with poor results may indicate energy deficit suppressing net synthesis
How to Improve It
3 Things to Remember
MPS requires two simultaneous signals: mechanical tension from resistance training and leucine from protein intake, both needed and neither sufficient alone.
The leucine threshold is 2.5 to 3g per meal, corresponding to 30 to 40g of complete protein; spreading the same total protein across many small doses underdelivers.
Training each muscle group twice per week doubles the total time MPS is elevated compared to once-weekly training, making frequency a key hypertrophy variable.
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